Xenobiotic chemicals can be metabolized by orans and tissues other than the liver. Additionally, the prostaglandin endoperoxide synthetase (PES) system is found in a number of organs and is not dependent on cytochrome P-450. The metabolism of aromatic amines, imidazoquinolines and other chemicals to mutagens in Salmonella tester strains was studied using PES. Specifically, the aromatic amines and imidazoquinolines being studied are those formed in beef during cooking, or from amino acids undergoing pyrolysis. All these chemicals are mutagenic following metabolism by liver, and other organ P-450-dependent mixed function oxidases.